Extreme ultraviolet light, e.g., electromagnetic radiation having wavelengths of around 50 nm or less (also sometimes referred to as soft x-rays), and including light at a wavelength of about 13.5 nm, can be used in photolithography processes to produce extremely small features in substrates such as silicon wafers. Here and elsewhere herein the term light will be used even though it is understood that the radiation described using that term may not in the visible part of the spectrum.
Methods for generating EUV light include converting a target material from a liquid state into a plasma state. The target material preferably includes at least one element, e.g., xenon, lithium or tin, with one or more emission lines in the EUV range. In one such method, often termed laser produced plasma (“LPP”), the required plasma can be produced by using a laser beam to irradiate a target material having the required line-emitting element.
The target material may take many forms. It may be solid or a molten. If molten, it may be dispensed in several different ways such as in a continuous stream or as a stream of discrete droplets. As an example, the target material in much of the discussion which follows is molten tin which is dispensed as a stream of discrete droplets. It will be understood by one of ordinary skill in the art, however, that other forms of material and delivery modes may be used.
Thus, one LPP technique involves generating a stream of target material droplets and irradiating at least some of the droplets with laser light pulses. In more theoretical terms, LPP light sources generate EUV radiation by depositing laser energy into a target material having at least one EUV emitting element, such as xenon (Xe), tin (Sn), or lithium (Li), creating a highly ionized plasma with electron temperatures of several 10's of eV.
The energetic radiation generated during de-excitation and recombination of these ions is emitted from the plasma in all directions. In one common arrangement, a near-normal-incidence mirror (often termed a “collector mirror” or simply a “collector”) is positioned to collect, direct (and in some arrangements, focus) the light to an intermediate location. The collected light may then be relayed from the intermediate location to a set of scanner optics and ultimately to a wafer.
The stream of droplets is generated by a droplet generator. The portion of the droplet generator that releases the droplets, sometimes referred to as the nozzle or the nozzle assembly, is located within the vacuum chamber. Considering the example of molten tin dispensed as a stream of discrete droplets, technical challenges arise in supplying target material to the droplet generator and in the recovery of the target material that is not vaporized. This is due in part to the fact that in operation the droplet generator must be maintained at a temperature above the melting point of the target material. It is also due to the fact that the interior of the droplet generator is maintained at pressure to expel the molten target material from the nozzle.
In general, it is possible to supply target material to the droplet generator by depressurizing and cooling the droplet generator, opening the droplet generator, loading solid target material into the droplet generator, closing the droplet generator, and repressurizing and heating the droplet generator. It can be appreciated that this method of supplying tin to the droplet generator can be quite time consuming and labor intensive. It also involves taking the droplet generator offline, resulting in significant downtime. This is especially troublesome when the design of the droplet generator is such that it must be reloaded frequently.
Also, it may be difficult to restart the droplet generator when the droplet generator is stopped and cooled down below the melting point of the target material. This is at least partly because the nozzle may have a very small orifice. Permitting the temperature of the nozzle to fall below the melting temperature of the target material may cause the target material in the nozzle to solidify. This may in turn cause or permit the formation of contaminant particles to form. These particles may precipitate out of the target material when the nozzle is reheated to re-melt the target material. Some particles may also be dislodged from surfaces upstream of the nozzle due to the effect of thermal contraction and expansion and associated mechanical stresses or by surface tension forces when the droplet generator is emptied. The particles can clog the nozzle thus making it difficult or impossible for the droplet generator to restart. Similarly, when the droplet generator runs out of target material, cooling the nozzle of the droplet generator can have a severe negative effect on nozzle integrity and can also make it difficult or impossible for droplet generator to restart.
Thus, cooling the entire droplet generator down to a temperature at which it can be handled safely and replenishing the target material may not always be a feasible method of reloading the droplet generator. Also, the necessity of replacing or repairing a droplet generator every time it needs to be shut down or the target material needs to be replenished results in significant downtime for the EUV light generation system and also limits the useful lifetime of the droplet generator.
Similar issues exist with respect to recovery of target material which is introduced into the chamber as droplets but which are not vaporized. This can occur, for example, in systems in which the droplet generator runs continuously and the generation of light is controlled by starting and stopping the laser which vaporizes the droplets.
Provision must be made for removing the unused target material from the vacuum chamber, preferably without breaking the vacuum in the vacuum chamber.
There is thus a need to supply the droplet generator with target material and remove unused target material that does not require excessive downtime. There is also a need to supply the droplet generator in a way that permits the droplet generator to restart reliably after a reload operation. It would also be advantageous to design a droplet generator which can be left in place and even remain in operation while it is being reloaded. There is also a need to be able to remove unvaporized target material quickly and efficiently.